1. Field of the Invention
The invention relates generally to switch mode power supply (SMPS) circuits and methods thereof, and more particularly to feed forward SMPS circuits and methods thereof.
2. Description of the Related Art
A switched-mode power supply, switch-mode power supply, or SMPS, is an electronic power supply unit (PSU) that incorporates a switching regulator, which is an internal control circuit that switches power transistors (e.g., MOSFETs) rapidly on and off in order to stabilize the output voltage or current. Switching regulators are typically used as replacements for the linear regulators when higher efficiency, smaller size or lighter weight is required. However, switching regulators are more complicated and their switching currents can cause noise problems if not carefully suppressed.
SMPS can be classified based on input and output waveforms, as a rectifier or off-line converter (AC in; DC out), a voltage/current or DC to DC converter (DC in; DC out), a frequency changer or cyclo-converter (AC in; AC out) and an inverter (DC in; AC out).
A buck converter is an example of a step-down DC to DC converter. The design of a buck converter is similar to that of a boost converter (i.e., a step-up DC to DC converter). A buck converter is relatively efficient (e.g., up to 95% for integrated circuits) compared to linear regulators.
A conventional buck converter typically includes two switches (e.g., a transistor and a diode) as well as an inductor and a capacitor for filtering of the output voltage ripple. A synchronous buck converter is a modified version of the basic buck converter circuit topology in which the diode is replaced by a second transistor. This modification is a tradeoff between increased cost and improved efficiency. Generally, the buck converter alternates between connecting the inductor to a source voltage to store energy in the inductor (“on state”) and discharging the inductor into a load (“off state”).
FIG. 1A illustrates a conventional synchronous buck converter 100. As shown in FIG. 1A, the conventional synchronous buck converter 100 includes a power supply voltage Vd, a first switch S1 and a second switch S2, an inductor L, and a capacitor C. Resistors rL and rc represent the effective series resistance (ESR) associated with the inductor and the capacitor respectively, whereas resistor R represents the load. Generally, the power supply voltage Vd corresponds to the DC input voltage of the synchronous buck converter 100, and the voltage Vo across the resistor R corresponds to the DC output voltage.
As discussed above, buck converters operate by switching between an “on state” and an “off state”. Referring to FIG. 1A, during the “on state”, switch S1 is closed and switch S2 is opened. In contrast, during the “off state”, switch S2 is closed and switch S2 is opened. Thus, the first and second switches S1 and S2 are always set to a different operating status. Accordingly, FIGS. 1B and 1C illustrate the on and off states, respectively, of the synchronous buck converter 100.
Conventionally, the switching of the first and second switches S1 and S2 is controlled by a feedback circuit (not shown) that monitors the output voltage Vo and compares the output voltage Vo with a reference voltage, which is set manually or electronically to the desired output. If there is an error in the output voltage Vo, the feedback circuit compensates by adjusting the timing with which the first and second switches S1 and S2 are switched on and off. For example, the switching regulator portion of the feedback circuit may be embodied as a proportional-integral-derivative controller (PID) controller. A PID controller is a well-known control loop feedback mechanism widely used in industrial control systems. Generally, a PID controller attempts to correct the error between a measured process variable (i.e., the feedback output voltage Vo) and a desired set-point (i.e., the reference voltage) by calculating and then outputting a corrective action, or compensation signal (i.e., adjustments to the S1/S2 switching) that can adjust the process accordingly.
Output voltage ripple is the name given to the phenomenon where the output voltage Vo rises during the On-state and falls during the Off-state. Several factors contribute to this including, but not limited to, switching frequency, output capacitance, inductor, ESR, load and any current limiting features of the control circuitry. At the most basic level the output voltage will rise and fall as a result of the output capacitor charging and discharging. Further, buck converters typically output the output voltage Vo to a load (i.e., resistor R). If the load current changes relatively quickly, the conventional buck converter 100 may have difficulty handling the load change (e.g., because the feedback circuit may over or under-compensate for the load change). Accordingly, if the load is changed, the output voltage ripple may increase, which degrades system performance.